If you have no source of fresh oxygen, you’re doomed.
Why? Because when you breathe in oxygen, you exchale carbon dioxide, which is something we cannot breathe. Carbon dioxide will fill the space, and since there’s no more oxygen, eventually you’ll die.
Anything below 70% oxygen saturation is life threatening
Humans and living animals are turning oxygen into carbon dioxide (CO2) all the time. CO2 is effectively a poison in high doses and must be gotten rid of in enclosed spaces. It would be replaced with oxygen from some storage tank.
Lack of oxygen will cause humans to initially behave strange, sometimes like they’re drunk… but eventually pass out, enter a coma, and cause brain damage. The brain needs oxygen or it will die surprisingly fast compared to the rest of the body.
This is why giving someone who’s heart isn’t beating CPR as soon as possible is so important, but in the CPR situation the air around us is clean. Inside a sealed up submarine, the air you bring with you is all you really have and it will eventually run out as long as living humans are onboard, and CPR won’t help here.
The oxygen is limited because it’s a enclosed submarine surrounded by water. It doesn’t have the ability to generate oxygen itself so it only has what it’s brought down with it.
The oxygen is running out because the humans are breathing it and converting it to Carbon Dioxide. It’s not like the oxygen is being destroyed or ‘lost’, it’s being bound up in a new molecule. When they refer to “oxygen” in this case, they are referring to what’s called “molecular oxygen” or the molecule O2. Once enough of the O2 has been breathed in and converted to CO2, the crew will lose consciousness and eventually die.
When you burn a piece of wood you’re taking oxygen from the air and combining it with the carbon in the wood to create heat (more specifically, turn the energy stored in the wood from “chemical bonds” to “heat”) and carbon dioxide (among other things).
The exact same thing happens inside your body, though much more slowly. When you breath in your body takes oxygen from the air and combines it with carbohydrates in your body to make energy and carbon dioxide.
So unless the air is being replenished in some way, merely breathing turns a greater and greater percentage of the oxygen in the air into carbon dioxide. Once the oxygen percentage of the air drops low enough your lungs can no longer pull in enough oxygen and you start suffocating. You body stops being able to use it’s stored fuel to power itself fully and, like a candle flame under a glass, you just grow weaker and weaker until *poof.* Gone.
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